Electrophotographic copiers and printers make use of toners for developing electrostatic latent images. The developed images are fixed on sheets or the like members to form permanent visual images. Broadly, there are two types of method for fixing the developed images: namely, a method called "heat fuse-fixing" in which resin particles in the toner are heated and fused on the sheet, and a method called "pressure fuse-fixing" in which resin particles are fused by application of pressure.
On the other hand, a device which is referred to as "heat roller fixing device" has been broadly used because of its superior characteristics, namely, stable fixing performance over wide speed range of developing machine, high thermal efficiency and safety. This device has a heat roller which is heated by a tungsten halogen lamp provided inside the roller. This constitution undesirably requires a large electric power consumption and long warm-up time. In addition, the roller temperature is lowered when many sheets are treated successively, because the heat output of the lamp cannot compensate for the temperature drop of the roller.
Thus, shorter warm-up time, reduced electric power consumption and smaller temperature drop are important requisites for the heat roller. More practically, the warm-up time is preferably 30 seconds, more preferably 20 seconds or shorter, while the electric power consumption is preferably less than 1 KW, more preferably about 700 W or smaller. It is also preferred that the roller temperature is stably maintained around 200.degree. C.
In order to develop a heat roller which can be heated up in the short time mentioned above, after an intense study, it was proposed that, from a view point of electric resistivity, a resistance film produced from an Ni-Cr alloy and a ceramic material by arc-plasma spraying method can suitably be used as a heat generator for this type of heat roller. (see copending patent application Ser. No. 686,850 assigned to the same assignee).
In the case of a heat roller which has a short warmup time, the roller temperature is raised to about 200.degree. C. in a very short time of 30 seconds or less as stated above. As a consequence, a considerably heavy thermal shock is repeatedly applied to the roller. Unfortunately, however, the above-mentioned resistance film prepared by arc-plasma spraying of the Ni-Cr alloy and the ceramic material, cannot withstand such a repetition of heavy thermal impact.
Another important requisite for the heat roller is that the roller exhibit a uniform temperature distribution over its entire surface. Generally, the heat roller tends to exhibit higher temperature at its mid portion than at both axial ends. This tendency is increased particularly when the resistance film has a positive temperature coefficient, i.e., such a characteristic that the electric resistance is increased in accordance with a temperature rise. Namely, in such a case, the portion of the resistance film on the mid portion of the roller exhibits a greater resistance than the film portions on both axial ends of the roller, so that the electric current which flows from one to the other axial ends encounters a greater resistance at the mid portion of the roller. Greater heat is generated at this portion of the roller thereby causing a further temperature rise at the mid portion of the roller. In order to attain a uniform temperature rise, therefore, it is preferred that the resistance film does not have a large positive temperature coefficient.
The resistance film could have a negative temperature coefficient, that is, such a characteristic that electric resistance decreases as temperature rises. In such a case, the heat generation is smaller at the mid portion of the roller than at both axial end portions of the same, and could theoretically contribute to a more uniform temperature distribution along the axis of the roller. However, when the roller temperature is still low, the resistance film exhibits a very large electric resistance such as to restrict the flow of the electric current, so that an impractically long time is required for heating up the roller. Thus, the use of a resistance film having a negative temperature coefficient does not meet the demand for shortening of the warm-up time. The control of the temperature of the resistance film is conducted by a control circuit which judges the film temperature by sensing the electric current, and varying the electric current in accordance with the measured temperature so as to maintain a constant film temperature. The resistance film having a negative temperature coefficient reduces its resistance when the temperature becomes high. If the electric resistance of a circuit for supplying the electric power is increased due to an unexpected reason such as an insufficient contact of terminals or contacts in the circuit, the temperature control circuit erroneously judges that the resistance film temperature has come down and operates to supply greater electric current to the resistance film. From the view point of stability of the temperature control, therefore, it is preferred that the resistance film have a positive temperature coefficient.